Electronic information devices have been digitalized in recent years, and the increase in driving frequency of microprocessor units (MPU) as a core part of these electronic information devices has been in progress, resulting in an increase in electric power consumption, and raising a remarkable problem of reliability caused by heat generation. As a countermeasure against this, attempts to reduce the driving voltage have been made. As a circuit for supplying a highly accurate electric power to the microprocessor, a DC-DC converter so called as a voltage regulator module (VRM) has widely been used. For an output-side capacitor, a large number of capacitors with low equivalent series resistances (ESR) are used for preventing any voltage drop. As a capacitor having this low ESR characteristic, a solid electrolytic capacitor using a solid electrolyte as an electrolyte has been practiced in use and widely used as a capacitor suitable for these purposes.
The increase in driving frequency of the microprocessor has been remarkable, however, with increasing the power consumption. In order to respond to that, the increase of the power supplied from the capacitor has been requested for preventing any voltage drop. In other words, a large power supply must be made in a short time, for which purpose the above-mentioned solid electrolytic capacitor is needed to not only be increased in capacity and decreased in size and voltage but also have an ESR characteristic lower than ever.
The solid electrolytic capacitor will be then described. A valve action metal foil such as aluminum, tantalum and niobium is subjected to an etching for increasing a surface-area thereof. Then, an etching is performed on the anode foil on which an anode foil oxide film was formed and also on the valve action metal foil such as aluminum, tantalum and niobium, so that a cathode foil is formed. The anode foil and the cathode foil is laminated through a separator comprising kraft paper, Manila paper, glass separator, or nonwoven fabric made of synthetic fiber such as vinylon and polyester. After an anode plug and a cathode plug are connected respectively to said anode foil and said cathode foil at optional positions, then the anode and cathode foils are rolled to form a capacitor element. A solid electrolyte is formed in this capacitor element, and the capacitor element is contained in a metal case. Then, the opening of the metal case is sealed tightly by use of a sealing resin comprising such resin as epoxy resin or by inserting a sealing rubber in the opening for a closing process.
In accordance with the solid electrolytic capacitor constructed as described above, used as the electrolyte is a solid electrolyte having a lower resistivity of not more than several tendsΩ-cm compared with the conventional electrolytic solution having a resistivity of the order of 100 Ω-cm. Accordingly, the solid electrolytic capacitor has a good ESR characteristic, as described above.
By the way, for the solid electrolyte, manganese dioxide with a resistivity of a few Ω-cm has been used conventionally. Later on, a solid electrolytic capacitor using polymers of TCNQ complex, polypyrrole and thiophene dielectric with resistivities of not more than 1 Ω-cm has been put into practical use. With a further increase in driving frequency of MPU, there has been a request for a capacitor having a smaller size, a larger capacity and a lower ESR. According to the studies by the inventors, such solid electrolytic capacitor has an insufficient effect of reducing the ESR thereof, despite the low resistivity of the electrolyte.
As described above, there is a limitation to the reduction of the ESR of the capacitor solely by lowering the resistivity of the solid electrolyte to thereby improve the solid electrolyte, and a further reduction of the ESR remains as a difficult problem.
The present invention was made to solve the above-described problem and provides a solid electrolytic capacitor which uses the solid electrolyte of a low resistivity and achieves a further lower ESR.